Molten lava, flash frozen upon explosive expulsion from the volcanic vent, instantly became what the Romans called “pozzolana”—pumice pozzolan, the key ingredient in Roman concrete. Roman structures such as aqueducts used volcanic ash as pozzolan in their concrete. Concretes using natural (pumice) pozzolan have proven to last thousands of years. Pozzolans fortify concrete, providing protection by mitigating various forms of chemical attack such as alkali-silica reaction (ASR), sulfate induced expansion, efflorescence, as well as rebar oxidation and debondment caused by the ingress of chlorides. Pozzolans also densify concrete, reducing porosity and permeability, thereby reducing chemical ingress and increasing long-term compressive strength and durability.
Fly ash, also known as flue-ash, is one of the residues generated in coal combustion and comprises the fine particles that rise with the flue gases. In an industrial context, fly ash usually refers to ash produced during combustion of coal. Fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys of coal-fired power plants. Depending upon the source and makeup of the coal being burned, the components of fly ash vary considerably, but all fly ash includes substantial amounts of silica (silicon dioxide, SiO2), alumina (aluminum oxide, Al2O3), iron oxide (Fe2O3), calcium oxide (CaO), and various metals.
In the past, fly ash was generally released into the atmosphere, but pollution control mandated in recent decades now requires that it be captured prior to release. Fly ash, particularly Class F fly ash, can be used as a pozzolan to enhance hydraulic cement or hydraulic plaster. Fly ash can be used as a replacement for some of the Portland cement content of concrete. Fly ash has historically been available at much lower cost than natural pozzolans as it is a waste material of coal-fired power plants with associated disposal costs.
Fly ash pozzolan, which is typically less expensive than a natural pozzolan, is generally used when chemical attack, such as alkali-silica reaction (ASR), is not expected to be severe. Furthermore, fly ash pozzolan is preferred when concrete with a low water-to-cement ratio is desirable. In general, fly ash creates less water demand than does a natural pozzolan. However, when chemical attack, such as ASR, is expected to be severe, a natural pozzolan would be a better choice.
Two classes of fly ash are defined by ASTM C618: Class F fly ash and Class C fly ash. The primary difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash. The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned.
The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash. This fly ash is pozzolanic in nature, and contains less than 20% lime (CaO). Possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in order to react and produce cementitious compounds. Calcium Hydroxide (Ca(OH)2), the major byproduct of the hydraulic reaction between cement and water, is the key chemical with which pozzolan reacts to form additional Calcium Silicate Hydrate (C-S-H), the binder in all Portland cement—based concretes.
Fly ash produced from the burning of younger lignite or subbituminous coal, Class C fly ash, in addition to having pozzolanic properties, also has some self-cementing properties. In the presence of water, Class C fly ash will harden and gain strength over time. Class C fly ash generally contains more than 20% lime (CaO). Unlike Class F fly ash, self-cementing Class C fly ash does not require an activator.
For the coal power industry, concrete has been a convenient market for fly ash. For concrete companies, fly ash has been a low-cost source of pozzolans. However, recently, a supply problem has started to emerge. Namely, due to increasing environmental regulations of power plants, the quantity and quality of fly ash has been decreasing. There is a declining availability of fly ash, particularly Class F fly ash, of suitable quality for use as a pozzolan in concrete. This situation is expected to worsen in the coming years.
In view of the challenges in two industries (electrical power and concrete), what is needed is a method to upgrade the pozzolanic quality of fly ash. In particular, it would be desirable to generate a remediating or beneficiating agent and process whereby non-certifiable, poor-quality fly ash, often referred to as non-spec fly ash, may be upgraded in quality in order to achieve certification under ASTM C618-12 and AASHTO M295.